1. Field of the Invention
This invention relates to a thermoplastic acrylic resin composition. More particularly, it relates to a thermoplastic acrylic resin composition which has excellent transparency, impact resistance, weather resistance and stress whitening resistance and is also low in temperature dependence of haze.
2. Related Art
It is common to improve impact resistance of thermoplastic resin by means of introducing an elastomeric substance into the resin. Diene elastomers are generally used for this purpose, but employment of such diene elastomers in resin products designed for outdoor use is undesirable because of the poor weather resistance property of diene elastomers.
Many studies have been made on the use of acrylic elastomers to provide impact resistance to the resin compositions without affecting their weather resistance property. Especially, many proposals involving the use of acrylic polymers having a multilayer structure, have been made.
For example, there have been proposed the blending of a particulate composite of a 3- or 4-layer structure and a thermoplastic polymer to improve impact resistance without impairing transparency (Japanese Patent Publication No. 27576/80); use of an acrylic polymer which is basically of a three-layer structure and has, between every adjoining layers, an intermediate layer having a concentration gradient which varies at an almost constant rate (Japanese Patent Application Kokai (Laid-open) Nos. 129449/76 and 58554/78); a polymer which is basically composed of three main layers and has at least one intermediate layer between the central soft layer and the outermost layer (Japanese Patent Publication No. 36646/84 and Japanese Patent Application Kokai No. 147539/82); a polymer having a soft-hard-soft-hard four-layer structure (Japanese Patent Application Kokai No. 94947/80). These proposals are certainly effective for improving anti-stress-blushing qualities but have the problems in transparency and/or whitening, high temperature dependence of haze.
There have also been proposed a method in which in the course of polymerization of the second layer (soft layer), at a point when the polymerization rate has reached 60 to 90% by weight, the monomer mixture for the third layer (hard layer) is added and polymerized (Japanese Patent Application Kokai No. 202213/84), and a method in which the monomer mixture forming the second layer (soft layer) is partly (15 to 30% by weight) left unreacted, and the third layer monomer mixture is polymerized to thereby form a layer structure which gradually changes in composition (Japanese Patent Application Kokai No. 27516/88). These methods are capable of improving impact resistance and weathering resistance but are unable to provide good transparency to the composition.
As seen from above, many proposals have been made on the particulate composites having a multi-layer structure for the purpose of improving the defective properties of acrylic resins without impairing their favorable characteristics. These proposals have indeed provided considerable improvements on transparency, anti-stress-whitening qualities and mechanical strength, but in regard to temperature dependence of haze, no satisfactory result was obtained.